Longwall mining of coal has increased substantially in the United States and currently accounts for over 50% of the underground mined coal. Increased production rates and high out-of-coal seam dilution (over 25%) in the United States and around the globe continue to generate dust control problems in mining areas. After a significant decrease in the number of incidents of coal worker's pneumoconiosis (CWP) over the last several decades, the number of reported cases in this decade is increasing. The primary cause of CWP is inhalation of respirable dust in a confined workplace; specifically, the inhalation of coal and quartz dust in a mine. The National Institute for Occupational Safety and Health (NIOSH) recognizes this disease as being severely disabling, potentially lethal, and entirely preventable through respirable (less than 10 micron) dust control. The typical protocol for prevention of this disease has been monitoring mine workers for symptoms of this disease and, once a CWP diagnosis has been made, moving the miner to a low-dust exposure job. Prevention of this disease through a significant reduction in mine workers' exposure to respirable dust is a high priority. Additionally, several mines face reduced dust standards due to high respirable quartz content in the dust.
In longwall coal mines, longwall shearers account for about 50% of the generated dust. Other important sources of dust include longwall supports, armored face conveyor, and stage loader. The shearer operators (SOs), roof support operators (ROs), and maintenance workers are likely to be overexposed to respirable dust. Dust control is a very important research problem in the USA and around the globe to meet the current US regulatory coal dust exposure requirement of 2 mg/m3 averaged over an 8-hour period. This problem has become even more critical with the implementation of new dust control regulations effective Aug. 1, 2014. The new regulations will now require meeting the new coal dust and quartz exposure requirements of 1.5 mg/m3 and 75 micro-gram/m3 averaged over the entire working shift effective Aug. 1, 2016. The working shift may be 8 hours but is not limited to this period. The working shift is up to the mine operator and may be between 4 hours and 16 hours. It is unanimously accepted among the professionals that meeting these requirements around the face area in longwall coal mines will be an extreme challenge.
Current technologies to control dust around shearer generally include water sprays behind each of the cutting bits on the two shearer drums and water sprays around the shearer drum. Optionally, crescent sprays may be provided at the end of the ranging arm and at the end of a shearer-clearer arm (SC) located about five (5) feet away and about parallel with the ranging arm. Chassis sprays that reside in a linear manner can also be provided and are located along the length of the chassis and directed at the freshly-mined face between the two shearer drums. The water volume used for sprays around each drum is about 30 gpm, with additional about 12 gpm for chassis sprays for total water volume of about 75 gpm. The peak production rate for a typical shearer is about 40 tons per minute for Illinois basin longwall faces, with about 65% of coal produced from the lead drum (or first drum) and 35% produced from the tail drum (or second drum). Another major problem facing longwall dust control is low residence time for dust to be wetted with water-based fluids (less than 5 sec) due to high air velocity (typically 400 ft./min to 1,000 ft./min) on longwall faces as compared to only about 100-200 ft./min for room-and-pillar coal mining. NIOSH research-based recommended locations and intuitively designed water sprays in the areas indicated above are being extensively used to control dust for the two shearer operators (SO) in the walkway, roof support operators (RO), and other operations support personnel in the face area. Based on an extensive review of pertinent literature and discussion with mine operators, there is no consensus on the type and location of sprays, volume of water and water pressure to be used in sprays. Although general guidelines have been developed by NIOSH researchers based on laboratory and field studies (see Controlling Respirable Dust on a Longwall Mining Operations, MSHA-CDC/NIOSH, Jim Rider, Jay Colinet—Slide presentation), mine operators tend to use their own intuitive thinking in using the spray system to suit their conditions.
Studies over the last several decades have attempted to identify sources of dust and solutions to the dust problems in mining environments. The conventional wisdom was presented by Chang and Zukovich (Cheng L and Zukovich P. P. 1973. Respirable dust adhering to run-of-face bituminous coals. Pittsburgh, Pa.: U.S. Department of the Interior, Bureau of Mines, RI 7765. NTIS No. PB 221-883.) They stated that a large amount of dust created does not become airborne and stays attached to the broken material. Therefore, spraying more water on the broken material tends to reduce dust. Adding water directly at the cutting picks that gets mixed with fragmented coal is more important than creating a shroud of water around the miner or shearer. Based on these observations, the conventional practice of mixing the water uniformly with broken coal was developed. This approach has been effective for coarser dust that are larger particles than airborne dust (I.e. larger than 10 microns). This approach is deficient for airborne dust because it blows the smaller particles into the environment.
Additional studies have observed that water can be used to control dust through the wetting of broken material and capture of airborne dust. (Kissel, F., “Handbook for Dust Control in Mining”, NIOSH, Information Circulation (IC 9465), 2003, pp. 131.) Although the methods of wetting broken material have been more uniform throughout the industry, unscientific approaches have been taken to the capture of airborne dust through the use of water sprays. This is most likely due to the problem and sometimes conflicting proposed solutions. It is suggested that a large number of smaller-volume sprays is better for dust control than smaller number of larger-volume sprays. U.S. Bureau of Mines concluded that many spray systems can create turbulent airflow in the face area that can result in rollback of dust. (Jayaraman, N, Fred N. Kissel, and W. E. Schroder (1984), “Modify Spray Heads to Reduce Dust Rollback on Miners,” Coal Age, June 1984).
Some research has proved valuable in the design of water spray systems. Courtney and Cheng concluded that typical water sprays operating at 100 psi do not capture more than 30% airborne dust in an open environment. (Courtney W. G. & Cheng L. 1977. Control of respirable dust by improved water sprays. In: Respirable Dust Control—Proceedings of Technology Transfer Seminars, Pittsburgh, Pa., and St. Louis, Mo., IC 8753, pp. 92-108. NTIS No. PB 272 910.) Furthermore, inappropriately designed sprays can displace dust clouds rather than wet or capture airborne dust. Reducing the water droplet size through the use of atomizing or fagging sprays may temporarily improve the airborne dust capture efficiency. However, small droplets tend to collapse/evaporate easily and release the captured dust. (McCoy J., Melcher J., Valentine J., Monaghan D., Muldoon T. & Kelly J. 1983. Evaluation of charged water sprays for dust control. Waltham, Mass.: Foster-Miller, Inc. U.S. Bureau of Mines Contract No. H0212012. NTIS No. PB83-210476.) Atomizing nozzles are most efficient in airborne dust capture followed by hollow cone, full cone, and flat sprays. Atomizing sprays can however get clogged frequently that can negatively affect equipment downtime and productivity. Hollow cone sprays are less likely to clog due to larger orifice area.
Nozzles operating at higher pressures are likely more efficient in the use of water while providing similar airborne dust capture efficiency. However, high-pressure sprays tend to disperse more dust and can create recirculation zones due to pressure differences. Therefore, their use is more appropriate in a relatively confined environment and does not work as effectively when there is air circulated in the mining area at high velocities.
In spite of considerable research done by the U.S. Bureau of mines (USBM), the National Institute of Occupational Safety and Health (NIOSH), and the industry over the last 40 years, there are significant limitations to the current practices. There is a need to improve the design concepts of sprays on and around the longwall shearer to control respirable dust (including quartz dust) exposure of workers around the mining face area.
One of the observable deficiencies in the present design includes poorly positioned sprays. For example, when the lead shearer drum is cutting near the top of the coal seam against the airflow, some sprays behind the cutting bits (adjacent to the face of the mine) tend to force and disperse dust toward an open area. This dust is entrained in the intake air traveling toward the drum and some of it travels toward the location of SOs and ROs and other personnel in the walkway in the face area. Furthermore, when the intake air traveling toward the tailgate intercepts the end of the shearer machine chassis, the air tends to rise due to upward pressure resulting from interaction with the armored face conveyor or AFC (traveling toward the stage loader). Some of the entrained dust travels to SOs and ROs located in the walkway in the face area.
The number of sprays mounted on the shearer-clearer (SC) arm is small and not located properly to provide effective hydraulic curtains to divide the air into fresh air stream and contaminated air stream. Furthermore, the length of the SC arm is not long enough to prevent dust-entrained air from reaching SOs and ROs.
The spacing between sprays on the chassis, and on the SC arm is generally small (3-5 inches). Therefore, there is considerable interaction between two adjacent sprays. These interactions (caused by different sprays hitting each other) results in water droplet size increase after interaction. Since the ability to capture dust requires that the water droplet size be near the size of the dust particle, this interaction significantly reduces the potential to wet the finer fractions of dust. Furthermore, most of the spray energy is dissipated in interactions rather than wetting the dust.
Another example of a deficiency is the use of improper pressures for several of the sprays. For example, several mines utilize high pressure sprays (150 psi and over) around the shearer drum. These sprays displace and disperse dust into the open space adjacent to the shearer drum. This dust is entrained in the incoming intake air and some of it can travel toward the SOs and ROs.
As such, there is a need to cure these deficiencies and maintain a relatively similar use of water consumption as the presently used designs. There is also a need to minimize the dust exposure by SOs and ROs in a longwall mining operation to levels below what are presently experienced. Furthermore, the required solution must negligibly impact the present coal extraction process that has been optimized for efficiently mining and extracting coal.